102 II. CHEMISTRY OF FATTY ACIDS AND GLYCEROL 



Glycerol is practically odorless, but has a sweet taste. The usual 

 article of commerce is a liquid at ordinary temperature; it is extremely 

 viscous. Pure anhydrous glycerol forms a crystalline solid which melts at 

 20 °C. It cannot be separated from water by distillation, since the poly- 

 hydric alcohol is steam-distillable. 



Glycerol decomposes slowly at the boiling temperature. However, the 

 rate of decomposition may be greatly accelerated when a dehydrating agent 

 such as acid potassium sulfate (KHSO4) is present on heating. Under such 

 conditions, two molecules of water are lost from each glycerol molecule, and 

 the characteristic product, acrolein (CH2:CHCH0), is formed. The lat- 

 ter compound can readily be recognized in extremely small amounts by its 

 penetrating and irritating odor. The formation of acrolein has been 

 widely accepted as a satisfactoiy qualitative test for free glycerol and for 

 fats. For more detailed information on the chemistry of glycerol, the 

 reader is referred to Lawrie.* 



Because glycerol is a triatomic alcohol, each molecule is able to combine 

 with three fatty acid molecules. Such compounds are referred to as tri- 

 glycerides. They comprise practically the entire bulk of the natural fats 

 and oils. However, mono- and diglycerides are also well known. These 

 are compounds in which only one or two fatty acids are combined with a 

 glycerol molecule. Although such compounds are present in an extremely 

 small proportion, if at all, in natural products, they can be readily syn- 

 thesized. They presumably confer special properties on fats, and are 

 sometimes added to them to render the fats adaptable for special uses. 

 They are present in the so-called "high ratio" shortenings. 



The wide variety of fats found in nature is possible because of the poly- 

 hydric nature of glycerol. In addition to the possibility of variation due 

 to the formation of mono-, di-, or triglycerides, a further isomerism of the 

 mono- and diglycerides may occur, depending upon the hydroxyl on 

 which the combination occurs. There are two different monoglycerides 

 (a or /3) and two distinct diglycerides {aa' and a0)', a and a' represent 

 the terminal carbons and jS represents the middle one. 



The "mixed" triglycerides offer the greatest opportunity for variations. 

 These are compounds in which more than one type of fatty acid is present 

 in the molecule. The properties of such mixed triglycerides may not 

 always be predictable from those of their component constituents. The 

 types and properties of some of these mixed triglj^cerides are discussed 

 later (Chapter III). 



Although the bulk of the fatty acids in nature are found in combination 

 with glycerol, many other compounds enter into stable union with them. 

 In these cases, also, the combination occurs through the carboxyl group 

 whereby the acidic properties of the fatty acid are dissipated. Examples of 

 such compounds are the higher alcohols which form waxes with fatty acids. 



